1. Field of the Invention
This invention relates to a method of forming a monolayer of particles, and to products formed thereby. It is particularly concerned with forming an ordered array of particles in a monolayer, but is not so limited. The monolayer of particles may be incorporated into a film. The invention is especially directed to films having anisotropic conductive pathways formed by ordered arrays of conductive particles, especially for use in interconnection technology in the electronics industry. However the invention also has utility in other fields of technology and may be applied to particles which are not electrically conductive.
2. Description of Related Art
Prior art relating to anisotropically-conductive adhesives and to the ordering of "magnetic holes" in ferrofluids is discussed in our International Patent Application WO 95/20820, the contents of which are incorporated herein by reference.
Japanese Patent Application J62127194-A of Fujikura Cable Works KK describes the production of anisotropic conductive solder sheets by forming an adhesive coating having a thickness of less than 10 micrometers on a support film, applying soft solder powder having a grain size of 10-50 micrometers onto the adhesive coating, and filling the spaces between granules of the solder with a plastic material. It is stated that the soft solder granules can be evenly dispersed in the plastic material on the film. However application of particles onto an adhesive film to which the particles adhere on contact is not likely to achieve satisfactory dispersion or ordering of the particles in the plane of the film.
EP 0 691 660 A1 of Hitachi Chemical Co. Ltd. describes an anisotropically electro-conductive film material produced by adhering electro-conductive particles to an adhering layer formed on a support and fixing the particles therein, and then introducing a film-forming resin incompatible with the adhering material between the electro-conductive particles, the film material having electro-conductivity only in the film thickness direction via the electro-conductive particles uniformly dispersed in the plane direction. The particles may be arranged in a grid or zig zag pattern in the plane by means of a film, net or screen having holes therein (hereafter called a "screen"), through which the particles are fixed on the adhering layer. The particles and the screen may be electrostatically charged with different electric charges. However the use of a screen having holes therein gives rise to a number of problems including the difficulty of producing and handling thin screens, and of making the desired patterns of holes therein. An individual screen would be required for each pattern. Also it would be difficult to ensure that all of the holes are filled by particles and to guard against clogging of some of the holes by the adhesive material. Removal of the screen may also cause disruption of the pattern. The use of electrostatic charging would be a complex procedure involving large electrical fields.
U.S. Pat. No. 5,221,417 Basavanhally describes the use of photolithographic masking and etching to form a matrix array of mutually isolated ferromagnetic elements. These elements are magnetized and a single layer of conductive ferromagnetic particles is adhered to an upper surface of each of the ferromagnetic elements, so that the conductive particles are in an array. The layer of particles is then contacted with a layer of soft adhesive polymer to cause penetration of the particles into the polymer. The adhesive polymer is then hardened to assure containment of the particles in the polymer. The adhesive polymer with the conductive particles therein is used for interconnecting conductor arrays. However this technique can only be used with conductive particles which are ferromagnetic. Such particles may be difficult to obtain in specific preferred shapes, sizes and types e.g. monodisperse spheres.
In WO 95/20820 (hereafter called "the first parent application"), we have described a composition comprising: (i) a ferrofluid comprising a colloidal suspension of ferromagnetic particles in a non-magnetic carrier liquid, and (ii) a plurality of electrically-conductive particles having substantially uniform sizes and shapes, dispersed in the ferrofluid.
Preferably the average particle size of the electrically conductive particles is at least 10 times (and more particularly 100 times, most preferably 500 times) that of the colloidal ferromagnetic particles. The non-magnetic carrier liquid may be curable or non-curable and may be selected from:
(i) a curable liquid composition, PA1 (ii) a mixture of a curable liquid composition and a liquid carrier in which the ferromagnetic particles have been suspended, and PA1 (iii) a non-curable carrier liquid, but if the carrier liquid is non-curable and the curable liquid composition is not present, the electrically-conductive particles have a latent adhesive property. PA1 (a) applying to one set of conductors a layer of an adhesive composition comprising a curable particle-loaded ferrofluid composition as described above; PA1 (b) bringing a second set of conductors against the layer of adhesive composition; PA1 (c) exposing the layer of adhesive composition to a substantially uniform magnetic field such that interaction between the ferrofluid and the electrically-conductive particles causes the electrically-conductive particles to form a regular pattern of particles each in electrical contact with an adjacent particle and/or with a conductor in one or both sets whereby conductive pathways are provided from one set of conductors to the other set, each pathway comprising one or more of the electrically-conductive particles; and PA1 (d) curing the composition to lock the pattern in position and to bond the conductors. PA1 (i) a solidifiable ferrofluid composition, the ferrofluid comprising a colloidal suspension of ferromagnetic particles in a non-magnetic carrier, and PA1 (ii) a plurality of electrically-conductive particles, dispersed in the ferrofluid,
In the first parent application we have also described a method of making an anisotropically-conductive bond between two sets of conductors, comprising:
However it may not always be convenient to install a means for creating a magnetic field at the location of assembly of two sets of conductors. Therefore in Irish Patent Application No. 950589 filed Aug. 1, 1995 (hereafter referred to as "the second parent application", the contents of which are incorporated herein by reference) we have described other ways of achieving the benefits of the invention of the parent application.
The second parent application describes an anisotropically-conductive film or a substrate having a surface coated with an anisotropically-conductive coating, said film or coating being formed by solidifying a composition comprising:
said electrically-conductive particles having been arrayed in a non-random pattern by application of a substantially uniform magnetic field to the composition in a liquid state and having been locked in position by solidification of the composition.
Furthermore, the second parent application describes a solid-form anisotropically-conductive film or a substrate having a surface coated with a solid-form anisotropically-conductive coating, said film or coating comprising a composition containing colloidal ferromagnetic particles and a plurality of electrically-conductive particles arrayed in a non-random pattern.
The term "ferromagnetic" as used in the second parent application and also used herein includes ferrimagnetic materials such as ferrites.
The term "solidifiable" as used in the second parent application and also used herein means capable of existing as a solid at ambient temperatures e.g. temperatures less than 40.degree. C., more usually 20-30.degree. C. Solidifiable compositions include curable compositions which cure to solid form by heat treatment or otherwise. The word "solid" as used in the second parent application means stable in shape and includes a gel or polymer network.
The inventions of the first and second parent applications are a significant breakthrough in the uniform dispersion of conductive particles and address the issue of particle aggregation and the consequences in fine pitch electronic interconnection (cf. U.S. Pat. No. 5,221,417 Basanvanhally). However the preparation of a curable particle-loaded ferrofluid adhesive composition requires a compromise between the desired ferrofluid character of the composition, including high magnetization saturation and low viscosity at room temperature for rapid ordering of the particles, and the desired adhesive character of the composition, including the use of medium to high molecular weight systems having relatively high viscosity, to impart good mechanical properties and functionality to the cured adhesive.
It is an object of the present invention to overcome some of the limitations in the inventions of the first and second parent applications.
It is also an object of the present invention to have wider utility in overcoming problems in the operation of the technology of EP 0 691 660 A1 and generally of arranging a monolayer of dispersed or ordered particles.
In respect of the foregoing, it is an object of the present invention to attain processes for the preparation of monolayers of dispersed or ordered arrays of particles as well as films containing the same which processes are easy, fast and employ readily available, easy to manufacture components and which allow for the reuseability/recovery of otherwise expensive and/or regulated materials.
It is also an object of the present invention to prepare stable monolayers of particles and arrays of particles and films containing the same which are free or substantially free of ferromagnetic particles.
It is also an object of the present invention to prepare films containing random and ordered arrays of particles with improved physical and performance characteristics, e.g., improved strength and/or adhesiveness as well as transparency or translucency, and the like.